The invention relates to the art of uniting parts of fractured bones by mechanical means, such as by plates or the like. More particularly, the invention relates to improvements in osteosynthetic force transmitting members or connectors which can be used, for example, to affix parts of broken bones to plates or other types of positioning means.
Osteosynthetic force transmitting members of the present invention can constitute screws, threaded inserts, pins, plates, rod-shaped load carriers, component parts of prostheses and/or surgical instruments (e.g., instrument, which can be used to drill holes or bores in selected portions of bones).
Osteosynthetic force transmitting members which can be constructed, assembled and used in accordance with the invention can be classified into several groups. A first group includes or embraces those osteosynthetic force transmitting members which are designed to constitute implants, i.e., devices which are or can be anchored directly in a bone. A second group embraces those members which constitute load bearing implants; i.e., devices which do not or need not penetrate directly into a bone. A third group embraces force transmitting members which are utilized by surgeons only in the course of an operation, e.g., to mechanically unite parts of fractured bones, and can constitute drills for making tapped bores or holes in fragments or selected portions of bones, flexible shafts or other instruments capable of being put to use preparatory to or in the course of a uniting operation.
A drawback of heretofore known osteosynthetic force transmitting members which belong to the aforediscussed groups, particularly but not exclusively to the first and second groups, is that their ability to transmit and/or stand forces is not sufficiently predictable. For example, problems can arise when a bone screw, a partially or fully threaded bone pin or another force transmitting member belonging to the aforementioned first group of such members is called upon to maintain plates, rods, bars and/or other osteosynthetic accessories in optimum positions for temporarily holding discrete bones or fragments of bones in a desired orientation relative to each other or to permanently affix discrete bones or fragments of bones to each other.
If the osteosynthetic treatment involves a bone having a hollow shaft which surrounds a medullary cavity for hematopoietic bone marrow, fatty tissue or a spongy part, it is often necessary to employ a substantially plate-like accessory which is provided with one or more holes for the shank or shanks of one or more bone screws which are driven into the bone. The parts of such bone assume the desired positions relative to each other when the plate-like accessory is properly held between a bone or an adjacent fragment of a bone and the head of the bone screw. As a rule, or in many instances, the head of each bone screw extends into a recess provided therefor in that side of a plate-like accessory which faces away from the bone. Each recess can be bounded by a concave (e.g., substantially hemispherical) surface complementary to the surface of that portion of a screw head which enters the recess when the fixing of the bone and the accessory relative to each other is completed.
The stability of the connection between several bones or between portions of a broken or otherwise damaged bone is ensured by causing the head of the screw to bear against a plate or another osteosynthetic accessory and by causing such accessory to bear against the adjacent bones or against the adjacent fragments of a particular bone. In many instances, that part of a properly inserted bone screw (namely a portion of the shank or stem) which has been caused to penetrate into a bone in order to maintain an accesory in requisite position is subjected (either exclusively or primarily) to longitudinal or tensional stresses. Bending stesses which develop when a bone screw is properly anchored in a bone and which would tend to bend or flex the inserted portion of the shank are taken up by the bone tissue which normally exhibits a certain amount of resiliency. However, that portion or section of the shank which extends from the bone is often subjected to highly pronounced mechanical stresses including bending or flexing and shearing stresses which can cause the screw to break in the region adjacent the head of the screw, namely between the external surface of the bone and the adjacent surface of an accessory in the form of a plate, rod, bar or the like. The provision of an external thread on the shank of a bone screw (such thread normally extends all the way from the tip of the shank to the head) also entails a weakening of the shank adjacent to the head, i.e., a weakening of that portion which is subjected to greatest bending and other stresses.
The likelihood of breakage of a bone screw adjacent the head, namely in the region closely adjacent the external surface of the bone and the confronting surface of an accessory generates numerous problems in actual use of such force transmitting members. If a break develops, it is normally very close to the exposed surface of the bone (i.e., at the location where the shank has penetrated into a bone or a bone fragment) so that it is difficult to extract the anchored portion of the shank from the bone. As a rule, such extraction must be preceded by time-consuming removal of those portions of the bone which are immediately adjacent the outermost part of the broken and concealed (embedded) portion of the shank. The removal of certain portions of the bone confining the broken off major portion of the shank of a broken screw is normally carried out by resorting to a tubular drill which must remove a relatively large amount of bone material in order to provide room for the application of tongs or another suitable tool (e.g., a cone) which is to extract the broken off portion of the shank from the bone. Such procedure results in pronounced and highly undesirable weakening of the bone because the thus weakened bone or bone fragment is likely to break at the locus of the hole or bore which confined the broken off portion of the shank. It is often necessary to transfer bone tissue from other portions of the weakened bone or from another bone in order to fill the hole or bore and/or the recesses or cavities formed by the drill and to thus increase the strength of the affected bone.
The problems which arise as a result of breakage of bone screws or analogous force transmitting members are even more acute if a force transmitting member (e.g., a pin or a screw such as a so-called pedicle screw) is used to penetrate into a vertebra forming part of the spinal column. The dimensions of a vertebra are such that it is not possible to employ more than a single pedicle screw or an analogous force transmitting member in order to establish a connection between the affected vertebra and a plate or another osteosynthetic accessory which is normally applied in order to take up forces acting in the longitudinal direction of the spinal column. Moreover, a pin or screw which is anchored in a vertebra is even more likely to break at the locus of exit from the respective vertebra because the bending or flexing stresses are increased due to the fact that a relatively long portion of the screw or pin between the vertebra and the accessory (be it a plate, a bar, a rod or the like) remains exposed, i.e., a substantial length of the force transmitting member (up to 2 cm in length) is not anchored in the vertebra and is not confined in the accessory but is exposed in a dorsal direction from the respective vertebra. This results in a long lever arm and greatly increases the likelihood of breakage of the force transmitting member between the vertebra and the accessory.
A force transmitting member which is used to affix a bone or a fragment of a bone to a plate or another accessory is likely to break on the additional ground that, at least in many instances, the area of contact between a bone or bone fragment and an accessory is very small. Therefore, only a small percentage of bending stresses acting upon the force transmitting member is converted into tensional stresses, i e., into stresses which are more likely to be withstood by the force transmitting member. All in all, breakage of a pedicle screw or another force transmitting member which is driven into a vertebra is much more likely to take place than the breakage of a bone screw or an analogous force transmitting member which is driven into a tubular bone or into a fragment of a tubular bone. It has been ascertained that approximately ten percent of force transmitting members which are driven into a vertebra are likely to break between the vertebra and the osteosynthetic accessory. Furthermore, the difficulties which develop preparatory to and during extraction of a broken portion of a force transmitting member from a vertebra are even greater than the aforediscussed problems arising in connection with the extraction of a broken portion of a bone screw from a tubular bone. It is normally necessary to remove a large portion of or even the entire root of the neural arc, i.e., of that part of a vertebra in which the shank of a pedicle screw or an analogous force transmitting member is anchored in the vertebra. This entails that it is no longer possible to reliably secure an osteosynthetic accessory to the thus damaged root. Therefore, it happens quite frequently that the surgeons decide to simply leave the broken off portions of force transmitting members in the respective vertebrae. The reason is that the removal of a broken off portion of a force transmitting member from a vertebra must be carried out with utmost care because the broken off portion of the force transmitting member is very closely adjacent to the spinal cord. As a rule, the distance of the broken off portion of a force transmitting member from the spinal cord is not more than 1-2 mm. On the other hand, a broken off force transmitting member which is left in the vertebra constitutes a potential source of problems at a later date. Therefore, it is highly desirable to remove the broken off portions of force transmitting members from the bones or bone fragments.
Conventional osteosynthetic force transmitting members and accessories are disclosed for example, in German patent application No. 36 30 863 A1 filed by Curt Kranz and published Mar. 17, 1988, and in German patent application No. 38 00 052 A1 filed by jurgen Harms et al. and published Jul. 13, 1989 (corresponding to U.S. Pat. No. 5,042,982).